Preparation of compounds of fluorine and carbon



M S vi955- M. w. FARLOW ET AL 2,709,186

PREPARATION OF COMPOUNDS 0F FLUORINE AND CARBON Filed Feb. 10, 1954 2Sheets-Sheet 2 PRODUCT OUT CU ELECTRODE HOLDER FLEXIBLE RUBBERCONNECTION GRAPHITE ELECTRODE GRAPHITE ELECTRODE 1-WAT ER JACKET CATHODEINVENTORS MARK WENDELL PARLO'W EARL LEONARD MUETTERTIES ATTORNEY UnitedStates Patent PREPARATECN F COMPDUNDS 0F FLUORINE AND CARBON lviarltWendeli Farlow, Wilmington, and Earl Leonard Muetterties, -i-llorkessin,Del, assignors to E. I. du Pont de Nemours a Company, Wilmington, Del.,21 corporation of Delaware Application February Ill, 1954, Serial No.409,484

12 Claims. (Cl. 260-653) This invention relates to a new process forpreparing compounds of fluorine and carbon. The present applica tion isa continuation-in-part of applicants copending application Ser. No.368,444, filed July 16, 1953, now abandoned.

Compounds containing only carbon and fluorine (hereinafter referred toas fiuorocarbons for the sake of brevity) are known to possessconsiderable usefulness in many fields of applied chemistry. Forexample, they have demonstrated utility as dielectrics, plastics,surface active agents, and the like. In particular, tetrafluoroethylenehas already achieved commercial success in the form of its polymer.However, the lack of an economical synthesis has prevented thedevelopment of really large markets for fiuorocarbons.

An object of the present invention is to provide a new process ofsynthesizing fluorocarbons. A further object is to provide an economicalprocess for such synthesis. Other objects will be apparent from thedescription of the invention given hereinafter.

The above objects are accomplished according to the present invention byreacting at a temperature of at least 900 C. carbon with a binaryfluoride of an element of group V-A of the periodic table or a binaryfluoride of one of those elements of group VIA having an atomic numberbetween 16 and 52, inclusive, and isolating the fluorocarbons formed.

The periodic table referred to in this discussion is that published inDemings General Chemistry, 5th ed., John Wiley and Sons, publishers.This table is used in many other reference books, such as the Handbookof Chemistry and Physics, 30th ed. (1947), published by the ChemicalRubber Publishing Company. Thus, it will be seen that the binaryinorganic fluorides adapted for use in this invention are those of thefollowing elements: nitrogen, phosphorus, arsenic, antimony, and hismuthfrom group V-A and sulfur, selenium and tellurium from group VLA.

The process of this invention can be carried out by passing thevaporized inorganic fluoride, if desired in a stream of inert gas suchas nitrogen, over carbon heated to a temperature of at least 980 C. Thereaction temperature can be as high as can practically be obtained byknown means. For example, the inorganic fluoride can be reacted with thecarbon electrodes of a carbon are (operated with either direct oralternating current), where the temperature is estimated to be in therange of 2500 C. to 3500 C.-r000 C. In fact, such an embodiment ispreferred since it tends to lead to high conversions to the extremelyvaluable tetrafluoroethylene. However, the reaction can also be carriedout by more conventional means, such as by passing the vaporizedinorganic fluoride through a tube containing carbon and heated bysuitable means to temperatures in the range of 9G0 C. to 2600 C.,preferably to a temperature of at least 1800" C. Another mode ofoperation consists in immersing the carbon arc in the liquid or moltenin organic fluoride. Still another mode of operation consists indropping the liquid or molten inorganic fluoride onto finely-dividedcarbon heated to the reaction temperature in a suitabiy arrangedreactor. Even more simply, a mixture of carbon and inorganic fluoridecan be heated in an open or closed vessel.

Any form of carbon, whether amorphous or crystalline, is suitable forthe purpose of this invention. Thus, there can be used coal, graphite,diamond, charcoal, the various forms of carbon black such as lamp black,acet ylene black, and bone black. At the lower temperatures, e. g., 9000-1500 C., the best results are obtained with active carbon, of whichmany Well-known varieties are available commercially. In general, activecarbon is very finely-divided, porous carbon having a total surface areaof at least 20 square meters per gram (Hassler, Active Carbon, ChemicalPublishing Co., 1951, p. 127). When using the carbon arc, the activityor state of subdivision of the carbon is apparently of no consequencebut the carbon must, of course, possess sufficient conductivity. Thecarbon need not be rigorously pure and it can, for example, contain thenormal amount of ash, e. g., from 0.5% to 4%, by weight, in the case ofmost active carbons. In certain cases, it is possible to use instead ofcarbon materials like calcium carbide which furnish carbon in situ inthis reaction.

The binary inorganic fluorides suitable for the purposes of thisinvention can be prepared by methods described in the literature, andsome of them are available commercially. Binary fluorides of theseelements in any valence state are suitable but it is preferredgenerally, in the case of the elements of group V-A, to use the binaryfluorides of these elements in their lowest valence state, for thereason that such binary fluorides: (for example, arsenic ll fluoride)are in many cases obtainable without having to resort to the use of freefluorine. Thus, they are much more readily accessible than the higherbinary fluorides of elements of group V-A.

Both reactants, that is, the carbon and the binary inorganic fiuoride,should preferably be substantially anhydrous, although the reaction cantolerate the presence of some Water. it is often desirable to dehydratethe carbon prior to reaction since carbon, especially of the ac tive orabsorbent variety, can contain signficant amounts of Water even at hightemperature.

While the relative proportions of the two reactants are not critical inso far as the course of the reaction is concerned, it is desirable foreconomic reasons to have the carbon present in excess, in order toutilize as much as possible of the more expensive binary inorganicfluoride. Thus, it is preferred to use the two reactants in suchproportions that there is present at least 0.25 gram atom, preferablybetween 1 and 5 gram atoms, of carbon per gram atom of fluorine. Therecan be used up to 20 gram atoms of carbon per gram atom of fluorine oreven more. However, in special procedures like the submerged arcoperation, the binary inorganic fluoride can be present in excess, atleast locally.

The reaction normally gives a mixture of fluorocarbons, the preponderantconstituents of which are carbon tetrafiuoride or tetrafluoroethylene,or mixtures of the two, with in general lesser amounts ofhexafluoroethane and octafluoropropane, and sometimes still lesseramounts of other saturated or unsaturated fluorocarbons. In addition thecrude reaction product contains in general some unreacted inorganicfluoride, which can be recycled, and the free element whose fluoride wasemployed, or compounds thereof. The fluorocarbons can be isolated, forexample, by passing the gaseous reaction mixture through cold condensersand fractionating the condensate through suitable distilling columns. Ifdesired, the gaseous reaction mixture can be circulated through coldbafiles to retain any reaction product which is solid at thattemperature, or it can be passed through liquid scrubbing solutions toseparate the unchanged inorganic fluoride and the element formed duringthe reaction. It is usually desirable to effect rapid cooling of thereaction products to avoid side reactions or polymerizations at the hightemperatures used. This is particularly the case when very hightemperatures, as in the carbon arc, are employed. In such cases, andespecially if tetrafluoroethylene is desired as the principal reactionproduct, very rapid quenching of the reaction mixture is recommended.

The reaction can be carried out at any desired pressure.

' Normally, atmospheric pressure is used but the pressure can be higheror lower.

The preferred means of carrying out the invention is by passing thebinary fluoride in gaseous state through a carbon are. This isillustrated in the accompanying drawing wherein:

Fig. 1 is a flow sheet illustrating the process; and

Fig. 2 is a section, more or less diagrammatical, of a carbon areadapted for use in the invention.

Fig. 3 is a section, more or less diagrammatical, of a second carbon arcadapted for use in the invention.

Referring to Fig. 1, the gas lines are of A (outside diameter) coppertubing. In a typical operation, the binary inorganic fluoride iscontained in cylinder 1. Valves 2 and 3 are closed, 5 and 6 are opened.The entire apparatus (with the exception of cylinder 1) is evacuated toremove air, trap 7 is cooled with liquid nitrogen, valve 5 is closed,argon (or other inert gas) is admitted through valve 3 to the desiredoperating pressure, and pressure controller 8 is set to maintain thatdesired pressure. The are 9 is struck, the reactant gas is passedthrough the are at the desired rate (flowmeter 10), and the product iscondensed in trap 7, except for a small amount of noncondensable gaswhich passes through controller 8 and pump 11 into gas reservoir 12.During operation, the arc inlet pressure (manometer 13) will beappreciably higher than the exit pressure (manometer 14) because of theconstriction involved in the arc passages. When it is desired to stopthe reaction the arc current is cut off, valves 2 and 6 are closed,valve 5 is opened, cylinder 15 is cooled with liquid nitrogen, trap 7 isallowed to warm to room temperature, and the volatile product isdistilled into cylinder 15. Finally, if desired, cylinder 15 can bepumped to remove traces of argon or other noncondensables, after whichthe cylinder valve is closed and the product is allowed to warm to roomtemperature.

Fig. 2 shows a carbon are suitable for use in this invention. This arecomprises the sections of copper tubing and 26, which serve as electrodeholders. Clamped to the section 25 is the cathode lead wire connection27 and mounted at the upper end of section 25 is the carbon electrode28. This electrode is suitably a graphite cylinder, in diameter by 3"long, with a 0.1" hole running longitudinally therethrough for passageof gases. The mounting of the electrode 28 in the copper tubing 25 is aconducting, gas-tight joint, suitably of copper foil wrapped around thegraphite cylinder, thus forcing the incoming gaseous inorganic fluoridethrough the longitudinal passage in the electrode 28. Similarly mountedin the section of copper tubing 26 is the electrode 29, likewiseprovided with a 0.1" hole running longitudinally therethrough. The anodelead wire connection 30 is clamped to this section 26.

'Encasing the electrodes is the glass water jacket 31 which is held inposition by the flexible rubber connections 32 and 33,. thereby forminga gas-tight compartment around the electrodes. There are also providedwater jackets 34 and 35 mounted on the sections of copper tubing 25 and26, respectively. The are is struck by contacting the electrodes 28 and29 manually through manipulation of one of the two flexible rubberconnections 32 and 33, care being taken to avoid contact withuninsulated portions of the apparatus. Thereafter, the electrode gap iscontrolled manually. A D. C. voltage is applied across the electrodes inthe conventional manner.

Fig. 3 shows a somewhat different carbon are that has given excellentresults when used in the process of this invention. This are is similarto that shown in Fig. 2 except for the sections of copper tubing holdingthe carbon electrodes and the electrodes themselves. In the are shown inFig. 3, the section of copper tubing connected to the cathode lead wireconnection 27 and holding the carbon electrode 41, is provided withperforations 42. The electrode 41, suitably made of graphite, is a solidcylinder approximately 4;" in diameter. The section of copper tubing 43is similar to the copper tubing 26 in Fig. 2 but it holds the graphiteelectrode 44 which is a hollow cylinder of 7 outside diameter and Ainside diameter. The opening in the electrode 44 is positioned so thatit is flush with the end of the electrode 41.

In the are shown in Fig. 3, the incoming gas flows out of theperforations 42 in the copper tubing 40, around the carbon electrode 41,and enters the hollow electrode 44, passing through the burning are atthis point. The reaction product passes through the electrode 44 and outthrough the copper tubing 43.

This arc can be operated with the electrode 41 plunged into the hollowelectrode 44 a short distance or the electrode 41 may be of the same orlarger diameter as the outside diameter of electrode 44 and separatedfrom it by a narrow gap. Also, the electrode 41 may be the anode and theelectrode 44 the cathode. It will be understood that specific dimensionsgiven above are merely for purpose of illustration and may be variedappreciably.

The invention is illustrated by the following examples in which allparts are by weight unless otherwise stated.

Example I A tubular nickel reactor was charged with 40 parts of carbonblack and heated at 1000 C. for 4 hours under a slow stream of drynitrogen. Glass traps cooled in liquid nitrogen were connected to theexit side and a slow stream of phosphorus III fluoride was passedthrough the reaction system at a temperature of 1075 C. to 1150 C., thegaseous products being condensed in the cold traps. In the course of 45minutes, 4 parts of phosphorus III fluoride were passed through thesystem and there was obtained 3.5 parts of condensate. This was shown byinfrared spectroscopic analysis to contain some unreacted phosphorus IIIfluoride, carbon tetrafluoride, and some carbon dioxide. The latter wasapparently formed by reaction of the carbon with traces of oxygenpresent in the phosphorus III fluoride.

Example 11 A slow stream of vaporized arsenic III fluoride was passedthrough an excess of carbon black heated at ture.

Example III Gaseous arsenic III fluoride was passed through the carbonare illustrated in Fig. 2 at a rate of 32 g./hr., an arc inlet pressureof 18 to 35 mm. of mercury, absolute, and an exit pressure of 7 mm.Fifteen to 20% of the arsenic III fluoride was converted tofluorocarbons in a single pass. The fluorocarbons were obtained in amolar ratio of -90% tetrafluoroethylene, 5% carbon tetrafluoride, and0.5% hexafluoroethane.

Example IV A carbon arc was operated under liquid arsenic III fluorideat 14 to 15 volts and 12 to 15 amperes, the reaction being carried outat atmospheric pressure. The liquid arsenic III fluoride boiledvigorously, and the reflux from an ice water-cooled condenser wasreturned to the reaction vessel. The uncondensed gases issuing from thecondenser contained, in addition to a small amount of unchanged arsenicIII fluoride, tetrafluoroethylene and carbon tetrafluoride in a molarratio of seven to one.

Example V Nitrogen ill fluoride containing about 6% by Weight of carbontetrafluoride, as determined by infrared analysis, was passed at therate of 61.8 g. per hour through the electric are illustrated in Fig. 3having graphite electrodes. The arc was operated at 26 volts and 18amperes and under a pressure of 1234 mm. of mercury. The duration of thereaction was minutes, during which 10.3 g. of the reactant gas (9.7 g.of NFs) was passed through the arc. The resulting condensable gas,consisting of unreacted nitrogen III fluoride and fluorocarbons, weighed8.8 g. It was found by infrared anlysis to contain, by volume, 30% ofcarbon tetrafluoride, of tetrafluoroethylene and 5% of hexafluoroethane.Allowing for the minor proportion of carbon tetrafluoride present in theinitial gas, calculations show that the condensable gas contained, byweight 27.8% of carbon tetrafluoride, 18.9 of tetrafluoroethylene and8.7% of hexaiiuoroethane (which corresponds to molar ratios of 56:33:11)and that the conversion of nitrogen III fluoride to fiuorocarbons was51.5%.

Example VI Phosphorus V fluoride was passed through the electric areillustrated in Fig. 3 at the rate of 54.4 g. per hour. The are wasoperated at 24 volts and 18-20 amperes and under a pressure of 6-35mm..of mercury. The weight of phosphorus V fluoride passed through thearc was 27.2 g. during a reaction time of minutes. There was recovered27.2 g. of reaction product containing, in addition to unchangedphosphorus V fluoride, 5% each, by volume, of carbon tetrafluoride andtetrafluoroethylene, corresponding to a 22.2% conversion of thephosphorus V fluoride to fluorocarbons.

Example VII A graphite tube packed with carbon black was placed in anickel reactor and a stream of dry nitrogen was passed through thereactor heated to 1000 C. for ten hours to dehydrate the carbonthoroughly and remove air. Glass traps cooled in liquid nitrogen wereconnected to the exit side of the reactor and a slow stream of sulfur Vifluoride was passed through the system for 3 /2 hours, during whichperiod the reaction temperature ranged from 950 C. to 1000 C. Thegaseous products were condensed in the cold traps. From 18 parts ofsulfur VI fluoride, there were obtained 13 parts of a mixture offluorocarbons. Infrared spectroscopic analysis showed that this mixtureconsisted preponderantly of carbon tetrafluoride and hexafluoroethane inthe molar ratio of about thirty to one.

Example VIII Gaseous sulfur Vi fluoride was passed through the carbonare illustrated in Fig. 2 at a rate of 47.5 g./hr., an arc inletpressure of 28-30 mm. of Hg, absolute, and an exit pressure of 6 mm. Theare was operated at 25 volts and 18 amperes. The product from a singlepass contained, by volume, parts of unchanged sulfur VI fluoride, 10parts of tetrafluoroethylene, 25 parts of carbon tetrafluoride, 5 partsof hexafluoroethane, less than 0.5 part of hexafluoropropylene, and 5 to10 parts of silicon IV fluoride formed through slight attack of theglass portion of the apparatus.

It will be understood that the above examples are merely illustrativeand that the present invention broadly comprises reacting at atemperature of at least 900 C. carbon with a binary fluoride of anelement of group V-A of the periodic table or a binary fluoride of oneof those elements of group Vi-A having an atomic number between 16 and52, inclusive, and isolating the fluorocarbons formed. 7

Suitable specific binary fluorides in group VA, other than those in theexamples, include arsenic V fluoride, antimony ill and V fluorides, andbismuth III and V fluorides. Of these binary fluorides, the mostpreferred one for use in this invention is arsenic llll fluoride whichis especially suitable for an economical, cyclic synthesis oftetrafluoroethylene since it is prepared from calcium fluoride, arseniclll oxide and sulfuric acid, and arsenic HI oxide is easily regeneratedby oxidation of the arsenic formed by reaction of arsenic lli fluorideand carbon.

Suitable specific binary fluorides in group Vl-A, other than sulfur Vifluoride, include sulfur IV fluoride, seleniurn IV and Vi fluorides, andtellurium'lV and VI fluorides. Of these binary fluorides, the mostpreferred one for use in this invention is sulfur VI fluoride. Sulfur VIfluoride is a gas and therefore well adapted to the process of thisinvention. Moreover, it is non-toxic and remarkably noncorrosive.

With regard to the reaction conditions, as already stated, the use of acarbon arc is the preferred method of carrying out the invention sinceit gives higher conversion and a higher yield of tetrafluoroethylenethan do other operating conditions. Therefore, the most useful mode ofpracticing this invention comprises contacting arsenic III fluoride orsulfur VI fluoride with a carbon arc and isolating the fluorocarbonsformed.

It will be apparent from the foregoing that an out standing advantage ofthis invention is that it provides a process whereby fluorocarbons, andespecially the highly valued tetrafluoroethylene, can be synthesizedeconomically.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that the invention is not limited to the specific embodimentsthereof except as defined in the appended claims.

The invention claimed is:

1. Process of synthesizing fluorocarbons which comprises reacting at atemperature of at least 900 C. carbon with a binary fluoride of anelement from the group consisting of the elements of group VA of theperiodic table and of those elements of group VIA of the periodic tablehaving atomic numbers between 16 and 52, inclusive, and isolating thefluorocarbons formed.

2. Process of synthesizing fluorocarbons which comprises reacting abinary fluoride of an element from the group consisting of the elementsof group V-A of the periodic table of those elements of group VI-A ofthe periodic table having atomic numbers between 16 and 52, inclusive,with the carbon electrodes of a carbon arc.

3. Process of synthesizing fluorocarbons which comprises passing in thegaseous state a binary fluoride of an element from the group consistingof the elements of group V-A of the periodic table and of those elementsof group VI-A of the periodic table having atomic numbers between 16 and52, inclusive, through a carbon arc and isolating the fluorocarbonsformed.

4. Process of synthesizing fluorocarbons which comprises passing in thegaseous state a binary fluoride of an element from the group consistingof the elements of group VA of the periodic table and of those elementsof group VlA of the periodic table having atomic numbers between 16 and52, inclusive, through a tube containing carbon and heated to atemperature of 900 C. to 2000 C, and isolating the fluorocarbons formed.

5. Process of synthesizing fluorocarbons which comprises reacting at atemperature of at least 900 C. carbon with a binary fluoride of anelement of group V-A of the periodic table and isolating thefluorocarbons formed.

6. Process. of synthesizing fluorocarbons which comprises reacting at atemperature of at least 900 C. car bon with arsenic III fluoride andisolating the fluorocarbons formed.

7. Process of synthesizing fluorocarbons which comprises reacting at atemperature of at least 900 C. carbon with a binary fluoride of anelement from the group consisting of those elements of group VI-A of theperiodic table having atomic numbers between 16 and 52, inclusive, andisolating the fiuorocarbons formed.

8. Process of synthesizing fluorocarbons which comprises reacting at atemperature of at least 900 C. carbon with sulfur VI fluoride andisolating the fluorocarbons formed.

9. Process for the preparation of tetrafluoroethylene which comprisesreacting carbon, at a temperature of at least 2500 C., with a binaryfluoride of an element 8 from the group consisting of the elements ofgroup V-A of the periodic table and of those elements of group VIA ofthe periodic table having atomic numbers between 16 and 52, inclusive,and isolating the tetrafluoroethylene formed.

10. Process according to claim 1 wherein carbon is reacted with thebinary fluoride at a temperature of at least 2500 C., the reactionproducts are very rapidly quenched and tetrafluoroethylene is isolated.

11. Process of synthesizing fluorocarbons which comprises reactingcarbon at a temperature of at least 900 C. with phosphorus V fluorideand isolating the flu0rocarbons formed.

12. Process of synthesizing tetrafluoroethylene which comprises reactingcarbon at a temperature of at least 2500 C. with phosphorus V fluoride,very rapidly quenching the reaction products, and isolating thetetrafluoroethylene.

No references cited.

1. PROCESS OF SYNTHESIZING FLUOROCARBONS WHICH COMPRISES REACTING AT ATEMPERATURE OF AT LEAST 900* C. CARBON WITH A BINARY FLUORIDE OF ANELEMENT FOR THE GROUP CONSISTING OF THE ELEMENTS OF GROUP V-A OF THEPERIODIC TABLE AND OF THOSE ELEMENTS OF GROUP VI-A OF THE PERIODIC TABLEHAVING ATOMIC NUMBERS BETWEEN 16 AND 52, INCLUSIVE, AND ISOLATING THEFLUOROCARBONS FORMED.